The invention relates to a method of positioning an actuator, in which method motion of the actuator toward a desired value, i.e. is controlled by a continuous control pulse. The continuous control pulse motion of the actuator is stopped when the position of the actuator is past a pulsing range limit, the pulsing range limit being so far from the desired value that the actuator stops before its position is substantially past the desired value. When the actuator has stopped, the position of the actuator is measured and if necessary a corrective run is performed by means of short control pulses of a standard length.
Actuators are used, for example, to control the flow of different media, such as water and steam, in different processes. The actuators control a valve, nozzle or other such element supplying the medium. The actuator can be a motor, cylinder or the like, which can be controlled in many different ways, for example mechanically, electrically, hydraulically or pneumatically.
The actuator is typically run as close to a desired value as possible, after which a verifying measurement is performed. After the verifying measurement, any necessary corrective runs are performed. This kind of control, however, is very inaccurate, and several corrective runs are needed, whereby the control becomes difficult to perform, and the actuator has to be moved back and forth and the clearances further increase the inaccuracy. The above control method is disclosed, for example, in Finnish Patent 85 731. The patent also sets forth the measurement of the clearance, but although the solution of the patent takes the clearance into account, the accuracy and speed of the control are not sufficiently good.
Another known method is to control the actuator to the desired value on a continuous basis, whereby the position of the actuator is measured simultaneously with the control. When the measurement shows that the actuator has reached the desired position, the motion of the actuator is stopped. Because the measurement software is slow, the measurement result, however, is obtained with a delay, whereby the actuator passes the desired value, and to correct the position, corrective runs need to be performed in the opposite direction. The clearances thus make the positioning inaccurate, and the control is complicated and difficult to perform.
German Offenlegungsschrift 41 03 422 teaches a solution in which the actuator is controlled by a continuous control signal when the position of the electropneumatic actuator is farther from the desired value than the certain pre-set limits. When the position of the actuator is closer to the desired value than said limits, the actuator is controlled by control pulses of a standard length and frequency. The length of the control pulses is arranged to be such that the position of the actuator will not pass the clearance i.e.g. a hysteres is produced range by the desired value because of the pulses. The supply of the control pulses to the actuator is continued until the position of the actuator is within the clearance. The solution enhances the accuracy of the positioning, but the actuator, however, is not controlled sufficiently quickly to the correct position in every application, since the motion effected by control pulses is slower than the motion effected by a continuous control signal.